In the boiling water reactor (BWR), output power can be controlled by changing a core flow and thereby changing a steam ratio (void fraction) inside a boiling reactor core.
However, it is known that depending on the core flow and other operating conditions, neutron flux distribution and liquidity in the reactor core are destabilized by delayed transportation of voids and a negative feedback effect caused by negative void reactivity coefficients in the reactor core.
There is concern that occurrence of such a nuclear thermal hydraulic destabilization phenomenon may result in considerable oscillation of output power and flow rate, which may deteriorate cooling characteristics in terms of fuel rod surface temperature and may damage the soundness of fuel rod cladding tubes.
Accordingly, in designing fuels and reactor cores for the boiling water reactor, the nuclear thermal hydraulic stability is analyzed to produce a design that gives sufficient margin to stability so as to prevent such an oscillation phenomenon from occurring in any of the expected operating ranges.
In such a range where deterioration in nuclear thermal hydraulic stability is expected, limited operation is preset for safety. Nuclear reactors of some types are provided with a safety setting so that in the unlikely event where the nuclear reactor reaches the operation limited range, output power is lowered by insertion of control rods and the like so that the nuclear reactor can get out of the operation limited range.
As the boiling water reactors are designed to have a larger size, a higher power density and a higher burn-up, their nuclear thermal hydraulic stability is generally lowered. However, measures for such boiling water reactors are not included in the above-stated safety setting.
In the case of operating the nuclear reactors which show good results in the U.S. at higher power, an operation control curve is expanded to a high-power side, which tends to increase a power/flow rate ratio and to deteriorate nuclear thermal hydraulic stability. In this case, according to the aforementioned safety setting, an operation control curve may possibly intersect a stability control curve in a low flow rate range. Consequently, an operable range on a low flow-rate side is largely limited, and operation at the time of activation and stop of the nuclear reactors may also be affected.
Under these circumstances, there are a large number of nuclear power plants which allow, from a viewpoint of Detect and Suppress, power oscillation phenomena while accurately detecting the power oscillation phenomena attributed to nuclear thermal hydraulic destabilization and suppressing the oscillations before the fuel soundness is damaged.
Accordingly, a power oscillation detection algorithm with use of dedicated detection signals for detecting the power oscillation phenomenon, which is referred to as OPRM (Oscillation Power Range Monitor), has been proposed (see, for example, U.S. Pat. No. 5,555,279 and U.S. Pat. No. 6,173,026).
As the performance of the boiling water reactors is reinforced to have a larger size, a higher power density, a higher burn-up and a higher power as described before, the substantial operating range is expanded, and thereby degree of allowances for nuclear thermal hydraulic stability is inevitably declined. In order to fully demonstrate an advantage of the reinforced performance of such boiling water reactors, it is required to further enhance accuracy and reliability in monitoring nuclear thermal hydraulic stability more than before.